1. Field of the Invention
The present invention relates to an optical recording medium in which a dipyrromethene metal chelate compound is used and which is recordable and reproducible at a higher density as compared with a conventional case.
2. Description of the Related Art
Presently, as an optical recording medium having a larger capacity as compared with a CD, a digital video disc (DVD) having a capacity of 4.7 GB has been developed and commercialized. The DVD is a ROM medium, and hence, recordable and reproducible optical record media having the corresponding capacity are desired. Among the DVDs, a write-once type is called a DVD-R.
In the DVD, in order to achieve a high-density recording, an oscillation wavelength of a laser beam is in the range of about 630 nm to 680 nm which are shorter wavelengths than in the case of the CD. As dyes for organic dye-including optical recording media which can be used within such short wavelengths, there have been suggested cyanine dyes, azo dyes, benzopyran-type dyes, benzodifuranone-type dyes, indigoid dyes, dioxazine-type dyes and porphyrin-type dyes in Japanese Patent Application Laid-open Nos. 74690/1992, 38878/1993, 40161/1994, 40162/1994, 199045/1994, 336086/1994, 76169/1995, 125441/1995, 262604/1995, 156218/1997, 193544/1997, 193545/1997, 193547/1997, 194748/1997, 202052/1997, 267562/1997 and 274732/1997. However, these conventional techniques have the problem of dye durability and particularly some problems peculiar to the uses of short wavelengths. For example, when a small pit is opened with a focussed laser beam, its periphery is widely affected, so that the largely expanded pit is inconveniently formed. This inconvenience causes the deterioration of a jitter and a radial cross talk. Conversely, in some of these conventional techniques, an extremely small pit is formed on occasion, and this inconvenience causes the deterioration of a modulated amplitude. In addition, when an organic dye having an unsuitable optical constant (a refractive index and an extinction coefficient) is selected and used for a recording layer at a desired laser wavelength, the deterioration of reflectance, sensitivity and the like occurs. Presently, these drawbacks have not been overcome at all yet.
An object of the present invention is to provide an optical recording medium which is recordable and reproducible by a short wave laser having a wavelength of 520 to 690 nm, is excellent in durability, and is suitable for a high-density recording.
The present inventors have already suggested a recordable and reproducible optical recording medium suitable for a high-density record, and in this kind of recording medium, a dipyrromethene metal chelate compound is used as an organic dye (Japanese Patent Application Laid-Open No. 226172/1998). The present inventors have further intensively investigated the optical recording medium in which the dipyrromethene metal chelate compound is used. As a result, it has been found that a selection of a dipyrromethene metal chelate compound having a substituent containing a vinyl group or a halogen atom, a substituent such as a specific aryl group or heteroaryl group into the xcex1-position of pyrrole ring, or copper or cobalt as a coordination metal in the above dipyrromethene metal chelate compound, and the use of two or more kinds of the selected dipyrromethene metal chelate compounds in a recording layer can provide an optical recording medium, in which the deterioration of reflectance and sensitivity caused by a drawback of the organic dye, i.e., a wavelength dependence of optical characteristics is inhibited and which is excellent in jitter properties and durability. In consequence, the present invention has been attained. That is to say, the aspects of the present invention are as follows.
1. The first aspect of the present invention is directed to an optical recording medium having at least a recording layer and a reflective layer on a substrate, wherein at least one dipyrromethene metal chelate compound represented by formula (1) is contained in the recording layer: 
wherein R1 and R9 are each independently an alkenyl group, an aryl group or a heteroaryl group; R2 to R8 are each independently a hydrogen atom, a halogen atom, or a substituted or unsubstituted alkyl, alkoxy, alkenyl, acyl, alkoxycarbonyl, aralkyl, aryl or heteroaryl group having 20 or less carbon atoms; and M is a transition metal.
2. The second aspect of the present invention is directed to the optical recording medium according to the above paragraph 1 wherein R1 in formula (1) is xe2x80x94CHxe2x95x90CHxe2x80x94R10, wherein R10 is a hydrogen atom, a halogen atom, or a substituted or unsubstituted alkyl, alkoxy, alkenyl, acyl, alkoxycarbonyl, aralkyl, aryl or heteroaryl group having 20 or less carbon atoms.
3. The third aspect of the present invention is directed to the optical recording medium according to the above paragraph 1 wherein R1 in formula (1) is a substituted or unsubstituted aryl or heteroaryl group having 20 or less carbon atoms, and M is copper or cobalt.
4. The fourth aspect of the present invention is directed to the optical recording medium according to the above paragraph 3 wherein R1 and R3 in formula (1) are simultaneously substituted or unsubstituted aryl or heteroaryl groups having 20 or less carbon atoms.
5. The fifth aspect of the present invention is directed to the optical recording medium according to the above paragraph 4 wherein R1, R3 and R9 in formula (1) are substituted or unsubstituted aryl groups having 20 or less carbon atoms, and at least one of the aryl groups is substituted by a halogen, or at least one of R2, R4 to R8 is a halogen atom.
6. The sixth aspect of the present invention is directed to the optical recording medium according to any one of the above paragraphs 1 to 5 wherein the recording layer further includes at least one other dipyrromethene metal chelate compound represented by formula (2): 
wherein R11 to R17 are each independently a hydrogen atom, a halogen atom, a nitro group, a cyano group, a hydroxyl group, an amino group, a carboxyl group, a sulfonic acid group, or a substituted or unsubstituted alkyl, alkoxy, alkylthio, aryloxy, arylthio, alkenyl, acyl, alkoxycarbonyl, carbamoyl, acylamino, aralkyl, aryl or heteroaryl group having 20 or less carbon atoms, and R15 and R16 may be bonded to each other to form an aromatic ring which may have at least one substituent; and Mxe2x80x2 is a transition element.
7. The seventh aspect of the present invention is directed to the optical recording medium according to the above paragraph 6 wherein the other dipyrromethene metal chelate compound is selected from compounds represented by formula (3): 
wherein R19 to R25 are each independently a hydrogen atom, a halogen atom, a nitro group, a cyano group, a hydroxyl group, an amino group, a carboxyl group, a sulfonic acid group, or a substituted or unsubstituted alkyl, alkoxy, alkylthio, aryloxy, arylthio, alkenyl, acyl, alkoxycarbonyl, carbamoyl, acylamino or aralkyl group having 20 or less carbon atoms; R18 and R26 are each independently an alkenyl group or an aryl group; and Mxe2x80x3 is copper or cobalt.
8. The eighth aspect of the present invention is directed to the optical recording medium according to any one of the above paragraphs 1 to 7 wherein at a laser wavelength of 520 to 690 nm, a refractive index and an extinction coefficient of the recording layer are 1.8 or more and 0.04 to 0.40, respectively.
9. The ninth aspect of the present invention is directed to the optical recording medium according to any one of the above paragraphs 1 to 7 which is recordable and reproducible by a laser beam selected within the wavelength range of 520 to 690 nm.
10. A dipyrromethene metal chelate compound represented by formula (4): 
wherein R27 to R35 are each independently a hydrogen atom, a halogen atom, or a substituted or unsubstituted alkyl, alkoxy, alkenyl, acyl, alkoxycarbonyl, aralkyl, aryl or heteroaryl group having 20 or less carbon atoms; and Mxe2x80x2xe2x80x3 is a transition element.
The employment of at least one dipyrromethene metal chelate compound of the present invention as a recording layer enables the provision of a write-once type optical recording medium which is very noticed as a high-density recording medium and which is recordable and reproducible by a laser beam having a wavelength of 520 to 690 nm and which is excellent in durability.